Multiwaveband polarimetric observations of NRAO 530 on parsec‐scale

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.52). 10/2010; 408(2):841 - 848. DOI: 10.1111/j.1365-2966.2010.17193.x
Source: arXiv

ABSTRACT We report on Very Long Baseline Array polarimetric observations of NRAO 530 at 5, 8, 15, 22 and 43 GHz made during one week in 1997 February. We present the total intensity, the fractional polarization and the electric vector position angle (EVPA) distributions at all these frequencies. A model fitting has been performed to the full polarization visibility data. From this, the fitted southernmost component A is confirmed as the core of the radio structure with relatively high brightness temperature and hard spectrum between 15 and 43 GHz in comparison with the central component B of dominant flux. The relatively high degree of polarization for the component A may arise from its complex radio structure, which can be resolved at 86 GHz. In contrast, the component B shows a well-fitted power-law spectrum with a spectral index of about −0.5 (f∝να), and a linear correlation between EVPAs and wavelength square with an observed rotation measure of about −1062 rad m−2, indicating its structural singleness. Assuming that the component B has a comparable degree of polarization without depolarization at these frequencies, the decrease in fractional polarization with wavelength mainly results from opacity and Faraday rotation, in which the opacity plays quite a large role. A spine-sheath-like structure in fractional polarization (m) is detected, covering almost the whole emission region at 5 and 8 GHz, with a degree of polarization relatively low along the jet spine, becoming higher towards two sides of the jet. The linear polarization at 5 GHz shows three separate polarized emission regions with alternately aligned and orthogonal polarization vectors down the jet. The polarization goes to zero between the top two regions, with the highest polarization level occurring at the top and bottom. The 5- and 8-GHz images show EVPA changes across the width of the jet as well as along the jet. These complex polarimetric properties can be explained in terms of either the presence of a large helical magnetic field or tangled magnetic fields compressed and sheared down the jet. These can be further determined by multifrequency polarimetric very long baseline interferometry observations with sufficient high resolution and sensitivity spanning an appropriate frequency range.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Using high frequency (12-22 GHz) VLBA observations we confirm the existence of a Faraday rotation measure gradient of ~ 500 rad/m^2/mas transverse to the jet axis in the quasar 3C 273. The gradient is seen in two epochs spaced roughly six months apart. This stable transverse rotation measure gradient is expected if a helical magnetic field wraps around the jet. The overall order to the magnetic field in the inner projected 40 parsecs is consistent with a helical field. However, we find an unexpected increase in fractional polarization along the edges of the source, contrary to expectations. This high fractional polarization rules out internal Faraday rotation, but is not readily explained by a helical field. After correcting for the rotation measure, the intrinsic magnetic field direction in the jet of 3C 273 changes from parallel to nearly perpendicular to the projected jet motion at two locations. If a helical magnetic field causes the observed rotation measure gradient then the synchrotron emitting electrons must be separate from the helical field region. The presence or absence of transverse rotation measure gradients in other sources is also discussed. Comment: Accepted to ApJ Letters; 11 pages, 4 figures (1 color figure)
    The Astrophysical Journal 05/2005; · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the results from the first quasi-simultaneous multifrequency (2.3, 5.0, 8.4, and 15 GHz) very long baseline interferometry (VLBI) observations of the compact steep-spectrum (CSS) superluminal source 3C 138. For the first time, the spectral distribution of the components within its central 10 milliarcsecond (mas) region was obtained. This enables us to identify the component at the western end as the location of the nuclear activity, assuming that the central engine is associated with one of the detected components. The possibility that none of these visible components is the true core is also discussed. The new measurements further clarify the superluminal motions of its inner jet components. The multifrequency data reveal a convex spectrum in one jet component, implying the existence of free-free absorption by the ambient dense plasma.
    The Astrophysical Journal 12/2008; 622(2):811. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a new method of extracting kinematic information from multiepoch very long base interferometry (VLBI) observations that relaxes the usual assumption that the jet is straight. Analyzing multiple observations of the quasar 3C 345 made by ourselves and by the Caltech group, we find that components C2 and C3 move along a common path and that their proper motions are consistent with a single constant pattern speed. With this assumption, we can then reconstruct the full three-dimensional trajectory of the jet. We find that the trajectory makes an initial angle theta approximately = 2 deg with the line of sight and is gently curving away from it. The Lorentz factor of the pattern speed is at least 11.8/h (H0 = 100 h km/s/Mpc). We then combine these kinematic constraints with the polarization information for component C3 and for the underlying jet obtained by VLBI polarimetry (Brown, Roberts, & Wardle 1994). We show that component 3C can be interpreted as a shock in a fluid jet with an ultrarelativistic equation of state. We explore the shock parameters that are consistent with the observations and determine the ranges of shock strength, thickness of the shocked region, upstream and downstream fluid velocities, and degree of order of the magnetic field in the underlying jet. We find that the shock speed is greater than that of the underlying jet (i.e., the fluid velocity is toward the nucleus in the frame of the shock) and that the shock must be rather weak. Also, there can be little particle acceleration in the shock apart from that due to adiabatic compression. This fast shock is in contrast to that required for the BL Lacertae object OJ 287, where the polarization, kinematic, and X-ray data show that the shock is slower than the underlying jet (Cawthorne & Wardle 1988). This difference may account in part for the slower superluminal speeds observed in BL Lacertae objects.
    The Astrophysical Journal 11/1994; 437:122-135. · 6.73 Impact Factor

Full-text (2 Sources)

Available from
Jun 1, 2014